alt Hacker Newsfrom the article (pay attention to the part in italics):
FLASH radiotherapy flips the conventional approach on its head, delivering a single dose of ultrahigh-power radiation in a burst that typically lasts less than one-tenth of a second. In study after study, this technique causes significantly less injury to normal tissue than conventional radiation does, without compromising its antitumor effect.
That phrasing isn't perfectly clear, as there's two things at play.
If you're delivering a large dose D all at once, FLASH spares normal tissue compared to conventional rate irradiations, with maintained anti-tumour effect.
But, you can instead deliver your treatment in a number of smaller doses, say n "fractions" of dose d. This also spares normal tissue (1). This latter approach - fractionation - is the way radiotherapy was delivered for most of its history. But at these low doses, FLASH sparing is small to negligible.
So, we have two demands in tension - and its unclear which is actually optimal. Some of the early results in FLASH showed huge sparing, but lots of more recent studies have shown more modest effects which may not be worth giving up benefits of fractionation for(2). And to date I think we have basically no meaningful in-human data to guide this, so there's still a lot of uncertainty.
1 - Fractionation also spares tumours, a bit, but you can offset this by increasing the total dose a bit and still see benefit.
2 - There is a general move to somewhat larger, fewer fractions even in normal radiotherapy, although almost all of these are still below the threshold where FLASH sparing is seen.